Lynn E. Dobrunz and Charles F. Stevens.
The Salk Institute
Synapses are dynamic; they respond to different patterns of input with short-term and long-term changes in synaptic strength. Facilitation of transmitter release and depletion of releasable vesicles are two types of short-term plasticity observed in hippocampal synapses. For central nervous system synapses in the hippocampus, studies of synaptic plasticity have investigated average properties of synapses by recording from populations of cells in the CA1 region or populations of synapses on a single CA1 cells. Synapses between hippocampal CA3 cells and CA1 cells, however, have been shown to contain as few as one active zone. This has enabled the investigation of the properties of individual release sites.
We have used stimulation of putative single release sites on CA1 cells in the hippocampus to show that significant heterogeneity exists in facilitation and depletion among synapses. In particular, the amount of paired-pulse facilitation is inversely related to the initial release probability. In addition, studies of synapse depletion were used to measure the size of the readily-releasable vesicle pool, revealing that the release probability at a synapse is directly correlated with the size of its readily-releasable vesicle pool.